Apache issue?

Hello everyone! Following this tutorial The perfect server - CentOS 6.2 I have installed ISPConfig 3 and configured 3 websites, but after a few hours websites are no more visible. The only way to make them visible again is to restart httpd.

My vps is running with 1Gb ram. System is using almost all ram, swap is working correctly.

I'm a little bit frustrated about this situation. Hope anyone can help.